amplitude of such vibration. The scratch of a pin at one end of a long
log can be heard by an ear applied to the other end of the log; but
every molecule in the log must move slightly; and there are all degrees
of movement between that visible to the eye, which we call mass motion,
and that called molecular simply because we cannot measure the amplitude
of the motion. We may, then, roughly divide all bodies into two classes,
as to their relations to sound,--such as re-enforce it, and such as
distribute it: the first depending upon the form of the body, as related
to a particular sound; the second independent of form, and responding to
all orders of vibrations. Air, wood, and metals belong in this latter
class. The common toy-string telegraph, or _lovers' telegraph_, is an
example of this class. Two tin boxes are connected by a string passing
through the middle of the bottom of each. When the string is stretched,
and a person speaks in one box, what is said can be heard by an ear
applied at the other. If the speaking-tubes be made about four inches in
diameter, and about four inches deep, they are capable of doing much
more service than is generally supposed to be possible. I know of two
lines, one of five hundred feet and the other of a thousand feet in
length, over which one can talk, and be heard with distinctness. In the
line of a thousand feet, the end of the tube is made of sheepskin
tightly stretched, and the line is made of No. 8 cotton thread. The
greater the tension, the better is the sound transmitted. The thread is
supported at intervals by running through a loop on the ends of cords
not less than three feet long, attached to supports. The thread pierces
the membrane, and is attached to a small button which is in contact with
the membrane. Wind and rain affect this line disadvantageously. The
other line of five hundred feet, between a passenger and a freight
depot, has the tube end covered with stretched calfskin. Instead of
thread, a copper-relay wire is employed (any small uninsulated wire will
do as well). This permits a good tension, and is unaffected by the
weather. One may stand in front of it about three feet, and converse
with ease, and in an ordinary tone. The wire is supported in loops of
string, as in the other.

Musicians have in all times employed various instruments for the
production of musical effects. Whistles made of bone were used by
pre-historic men, some of them having finger-holes so that